In satellite-based very-high-throughput systems, the feeder link between the ground feeder station and the satellite is a bottleneck because all the digital data intended for users and generated by users pass over this feeder link connecting the feeder station to the satellite.
Thus, for example, a satellite-based very-high-throughput system operating in the Ka band (20/30 GHz) having a zone of coverage made up of 200 spot beams, with a bandwidth of 1.4 GHz allocated to each spot, is able to transmit, to users, on average more than 500 Gbits/second (average spectral efficiency of 1.8b/s/Hz). The feeder link must therefore allow more than 500 Gbits/second to be transmitted to the satellite, this being a considerable throughput for a radiofrequency link. The frequency bands allocated to the feeder links of geostationary satellites by the International Telecommunication Union (ITU) do not exceed 2 GHz in the Ka band (20/30 GHz) and 5 GHz in the Q/V band (40/50 GHz).
There is therefore a problem to be solved with regard to decreasing the data flux that transits over the feeder link, in order not to exceed the available throughput resources.
One known way of solving the drawback related to excessive demands on the feeder link is based on increasing the bandwidth and/or data throughput of this link. This increase in resources may, for example, be achieved via a technique in which frequencies and polarizations are reused by several feeder stations that are distant from one another. Thus a satellite-based very-high-throughput system requiring a throughput of more than 500 Gbits/second over a Q/V-band feeder link may make use of more than about thirty feeder stations distributed over the zone of coverage.
Moreover, Q/V-band radiofrequency links through the atmosphere are very sensitive to the effects of attenuation and distortion of the signals due to bad weather (rain, snow, hail, etc.) and therefore these radiofrequency links are regularly inoperative. It is necessary to implement a site-diversity technique i.e. to install back-up and redundant feeder stations and to connect them to the other feeder stations via a terrestrial network. When a radiofrequency link generated by a given feeder station is inoperative because of bad weather, a back-up and redundant feeder station where the weather is good is activated to provide the feeder link to the satellite.
This solution is very expensive in terms of the initial investment required to install a great number of feeder stations and to connect them together via a fibre-optic network and in terms of operation and maintenance costs.
Another way of avoiding overuse of the feeder link is to use a web-caching technique to decrease the need in terms of data throughput of this link. All the content, and in particular videos, requested by the users of a satellite-based very-high-throughput system transits over the feeder link. Thus, the most popular content is retransmitted over the feeder link as many times as it is requested. Prospective studies show that video traffic will represent 70 to 80% of all Internet traffic by 2020. Measurements on terrestrial web-caching systems have shown that caching the most popular videos allows server traffic to be decreased by at least 50%. Transposing this result to a satellite-based very-high-throughput system, a 50% decrease in the need for feeder-link throughput is expected with the implementation of a web-caching system.
A web cache may be implemented in three locations in the system: in the user terminal, in the feeder station and in the satellite. The installation of a cache server in user terminals has a very small impact on the data throughput of the feeder link and it is, in general, already implemented in satellite-based very-high-throughput systems. The installation of a web-caching system in a feeder station has no impact on the throughput of the feeder link. Only the installation of a web-caching system on board the satellite allows decreasing the needs of data throughput of the feeder link.
Implementation of this solution, consisting in installing a web-caching system on board a satellite, has been described in U.S. Pat. No. 6,697,850 “Satellite-based communications system having an On-board Internet Web Proxy cache”. This solution is very complex and expensive because it requires additional and specific hardware to be installed on board the satellite, such as multi-carrier demodulators for detecting requests generated by user terminals, a proxy server for analysing the requests, a cache server and its memory for storing and managing the content, modulators and multiplexes for transmitting, to the users, the cached content, and high-throughput demodulators for receiving, via the feeder link, the content to be cached. The deployment of such a solution has another drawback, namely it requires a new type of more complex and more expensive user terminal to be implemented.